WASHINGTON (REUTERS) - United States Republican President Donald Trump appears to have narrowed in on his preferred foe for the 2020 election: Mr Hunter Biden, the son of leading Democratic presidential contender Joe Biden.

A frequent but silent presence in his father's decades-long political career, Mr Hunter Biden, 49, had never sought the limelight as he struggled in private with drug and alcohol addiction.

The lawyer and lobbyist has now seen his profile skyrocket as the frequent target of Mr Trump, who is seeking to depict him as the embodiment of corruption in an effort to sully his father's presidential campaign.

It was always Mr Biden's eldest son, Beau, who lived in the public eye. A former federal prosecutor, Mr Beau Biden went on to become the attorney general of Delaware, an assumed stepping-stone for a political career that could have one day made him governor, senator or even a presidential contender.

He died in 2015 after a fight with brain cancer.

Mr Hunter Biden also became a lawyer, but instead of public service, he went to work for MBNA, which at the time was one of the largest employers in Delaware, a credit-card company that hired him for a lucrative in-house job.

He would later become a Washington lobbyist, where his clients included universities and tech companies, like the now-defunct Napster. He stopped lobbying when his father got elected vice-president in 2008.

Mr Trump points to Mr Hunter Biden's role after he left lobbying, when he started a consulting shop meant to help foreign companies, and his work in China and Ukraine while his father was vice-president under Democratic president Barack Obama.

On Thursday (Oct 3), Mr Trump upped the pressure, publicly calling on Beijing and Kiev to launch investigations into Mr Hunter Biden's work - repeating accusations of corruption that he has made with no evidence.

It was Mr Trump's July call with Ukraine's president urging that country to investigate the Bidens that prompted Democrats in the US House of Representatives to launch a formal impeachment inquiry into Mr Trump.

The President's call on Thursday for China to investigate came only moments after discussing how he could exert pressure on that nation to approve a trade deal.

PROMINENT RIVAL

Mr Joe Biden not only leads most opinion polls among the 19 Democrats seeking their party's presidential nomination but also bests Mr Trump in several polls measuring a hypothetical match-up in November 2020.

His standing in the polls, he argues, is what is provoking Mr Trump to levy increasingly aggressive and public attacks against both him and his son.

The Biden campaign accused Mr Trump of abuse of power on Thursday following the President's suggestion that China investigate Mr Biden, but did not mention Hunter in the statement.

It did not respond to a Reuters request for comment.

But the turmoil has brought to the forefront the personal details of Mr Hunter Biden's life that can easily be assumed he would have rather remained private.

In an interview with The New Yorker magazine earlier this year, he detailed his repeated attempts to kick a drug and alcohol addiction.

He discussed missing his father's campaign launch rally in Philadelphia last May - disclosing that in the same week, he had proposed to and married a woman he had known only a few weeks.

"I've never missed a rally for my dad. The notion that I'm not standing next to him in Philadelphia, next to the Rocky statue, it's heartbreaking for me," Mr Hunter Biden told The New Yorker later.

"It's killing me and it's killing him. Dad says, 'Be here.' Mom says, 'Be here.' But at what cost?"